1. Field of the Invention
Embodiments discussed herein relate to a semiconductor device fabrication method.
2. Background of the Related Art
In recent years semiconductor substrates made of silicon carbide (SiC) have been used in place of semiconductor substrates made of silicon (Si) in order to realize high performance semiconductor devices.
Ion implantation is performed in processes for fabricating such semiconductor devices in order to electrically arrange functional regions by implanting impurities and realize operation as a semiconductor device. However, simply performing ion implantation causes a crystal defect or creates an amorphous region in which crystallinity is degraded. As a result, a function as a semiconductor device may not be fully realized. Therefore, there is a need to perform heat treatment on a crystal defect caused by ion implantation or an amorphous region in which crystallinity is degraded by ion implantation. By doing so, crystallinity is recovered and electrical activation is performed. If silicon carbide is used, it is difficult for impurities to diffuse. In addition, silicon carbide has polytypes (crystal polymorphism) which differ in physical property such as forbidden band width or impurity level. In order to prevent a polytype from mingling and realize good recrystallization, ion implantation is performed in a high-temperature environment compared with silicon. Therefore, for example, a heat-resistant oxide film is used as a mask film in place of photoresist used in the case of silicon (see, for example, Japanese Laid-open Patent Publication No. 2006-324585).
By the way, in order to reduce the costs of semiconductor device fabrication, a large-diameter semiconductor wafer is used. By doing so, the number of semiconductor chips obtained is increased.
Furthermore, with semiconductor elements, such as an insulated gate bipolar transistor (IGBT), in which current flows in the vertical direction, there is a need to make a semiconductor wafer thin for maintaining or improving the performance.
However, if ion implantation is performed on a thin large-diameter semiconductor wafer (semiconductor substrate) made of silicon carbide in a high-temperature environment by the use of a silicon oxide film or the like formed by a chemical vapor deposition (CVD) method or the like as a mask film, the mask film cracks or peels. A semiconductor substrate made of silicon carbide or the like and an oxide film differ in linear expansion coefficient. Residual stress is created in a stage in which the oxide film has been formed and patterned. When they are heated at a high temperature (300° C. to 500° C.) at ion implantation time, a mask film, which is the oxide film or the like, cracks or peels or a crack appears or peeling occurs at an interface between the mask film and the semiconductor substrate, because of the difference in linear expansion coefficient between silicon carbide and the oxide film. Furthermore, boron (B), aluminum (Al), phosphorus (P), nitrogen (N), arsenic (As), and the like are implanted in a semiconductor substrate made of silicon carbide or the like. As a result, a region in which ions are implanted and a region in which ions are not implanted differ in linear expansion coefficient. Accordingly, stress also acts on a mask film which is an oxide film or the like, which is formed over the semiconductor substrate, and which is patterned. As a result, the mask film cracks or peels.
Furthermore, in order to reduce crack formation or peeling, the thickness of the oxide film may be decreased. However, if silicon carbide is used, it is difficult for impurities to diffuse. Accordingly, there is a need to implant impurities more deeply at energy higher than acceleration energy for ion implantation used for conventional silicon semiconductor devices. In this case, it is desirable that the thickness of the mask film, which is a silicon oxide film or the like, be at least 0.5 to 3 μm. It is desirable not to reduce the thickness of the mask film beyond this limit.
Therefore, because of thermal stress created due to the difference in linear expansion coefficient between the semiconductor substrate and the oxide film, the oxide film peels off the semiconductor substrate or a crack or the like appears in the oxide film. As a result, the oxide film does not function as a mask film. Accordingly, ion implantation is not performed properly on the semiconductor substrate and the characteristics of a semiconductor device may deteriorate.
Furthermore, if a large-diameter semiconductor wafer, more particularly a semiconductor wafer having a diameter of 4 inches or more is used, these problems become significant.